MassMailer is a tool which allows you to do Special:EmailUser en masse to a list of users on a specified wiki.
It works on behalf of a Wikimedia registered user, using OAuth to authenticate that sending user. This tool must still be configured as an authorized OAuth client application in preferences of the sending user, and cannot be used by unauthenticated third parties without using their own registered account on a Wikimedia wiki. That authenticated user must not be blocked against using the Special:Emailuser on the target wiki, and some recipients may still reject the mails from that user. To use this application, senders must respect the relevant site policies of every target wiki about email usage.
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